Toxicity of many xenobiotics has been associated with lipid peroxidation (LPO) caused by reactive oxygen species (ROS) generated during their metabolism. Among the antioxidant enzymes, only glutathione peroxidases (GPxs) are known to provide protection against LPO by reducing lipid hydroperoxides. The ?-class glutathione S-transferases (?-GSTs), also show GPx activity towards lipid hydroperoxides, but the physiological role of this activity is not understood. Our preliminary studies show that human hGSTA1-1 and hGSTA2-2 can reduce membrane phospholipid hydroperoxides (PL-OOH) in situ by their GPx activity. Over expression of these enzymes in K562 cells attenuates oxidant (H202 or xenobiotic) induced LPO and cytotoxicity. The over expression of another ?-GST, hGSTA4-4 which detoxifies the end- product of LPO, 4- hydroxynonenal (4-HNE), by conjugating it to glutathione, also protects against oxidant toxicity. We hypothesize that GSTs provide a second line of defense against ROS and act as antioxidant enzymes, which protect cells against toxicity of oxidants/xenobiotics by attenuating LPO. The following specific aims are proposed to test this hypothesis: 1. The physiological significance of the ?-GSTs, GSTA1-1 and GSTA2-2 will be studied by determining their kinetic properties towards LPO products, PL-OOH and 4-HNE. Their contributions in the reduction of PL-OOH in rat and mouse liver will be determined and compared with those of the seleno GPxs. We will examine if the cells over expressing these enzymes are protected against H202 or oxidant xenobiotics (e.g., doxorubucin, CCI4) induced activation of c-Jun N-terminal kinase (JNK), caspase 3, and subsequent apoptosis. 2. Cells will be transfected with the ?-GST isozyme, GSTA4-4, which detoxifies 4-HNE and the toxicity of H202 and oxidant xenobiotics which induce LPO will be compared in the transfected and control cells. Xenobiotics, H202, xanthine/oxidase, DOX induced toxicity and apoptosis will be compared in the control and transfected cells to delineate the role of 4-HNE and GSTA4-4 in oxidative stress mediated signaling. 3. We will examine whether the toxicity of oxidants is enhanced in GSTA4-4 knock out mice because of their inability to detoxify 4-HNE. Since GSTs can be induced by non-toxic micronutrients, these studies will help in devising strategies for negating the toxicity of environmental chemicals and chronic oxidative stress, which leads to age, related degenerative disorders. [unreadable] [unreadable] [unreadable] [unreadable] [unreadable]